This invention relates in particular to a method for improving the quality of an audio signal. The invention also relates to an output stage with which the quality of an audio signal can be improved. In addition the invention relates to a mobile wireless terminal (also known and hereinafter referred to as mobile station) by means of an arrangement made in which the quality of an audio signal can be improved.
Several different types of amplifier have been developed for audio signal amplification. Audio amplifiers are conventionally classified and the different classes are denoted by letters A, B, AB, C, and D. It is typical of a conventional class A amplifier that it operates in the linear range whereby it is especially advantageously used in audio applications. The drawback of the class A amplifier is that it has a low efficiency for which reason it is seldom used in audio output stages. A class B amplifier has a high efficiency because of the typical push-pull operation, but in order to reduce the crossover distortion a class AB amplifier is almost always required which gives lower efficiency. The class AB amplifier represents a good compromise between the class A and class B amplifiers but it requires that the operating point of the amplifier stage does not drift according to temperature, for example. Therefore, a class C amplifier used in radio-frequency applications uses an operating point that makes the transistor non-conductive in the absence of an input signal, but in order to reduce distortion, resonating circuits are needed at the output. On the other hand, the efficiency and linearity of a class D amplifier are in principle good without operating point adjustment since the output stage acts only as a switch. A class D amplifier becomes fully conductive when an input signal is applied. A typical bridge-connected class D amplifier requires only a third of the operating power of a corresponding class AB amplifier with equal output power and distortion factor. A disadvantage is that the switching frequency of a class D amplifier has to be filtered from the output signal by means of a low-pass filter, but if the switching frequency is high enough the filtering will be simpler. A class D amplifier is typically used to amplify an analog signal, in which case the input signal of the amplifier is analog and the possible feedback signal is also a continuous analog signal. The feedback signal is usually taken from the low-pass filter output. In some recently developed class D output stages, the operation of which is based on pulse width modulation, as disclosed e.g. in the U.S. Pat. No. 5,594,386 (Dhuyvetter), the analog feedback signal is generated by integrating the output pulses prior to possible low-pass filtering. However, the feedback method disclosed does not allow for pulse amplitude variation caused e.g. by fluctuations in the operating voltage and therefore is not able to improve the quality of an audio signal by eliminating the resulting distortion. Thus it would be advantageous to use a class D amplifier directly in the processing of digital audio signals and, especially advantageously, the quality of the signal could be improved at the same time.
In modern mobile stations the digital audio signal is typically first converted into an analog signal, then amplified and only then fed to the loudspeaker. This is, however, impractical as such an arrangement involves a lot of wasted power. It is, indeed, more practical to feed direct to the loudspeakers the analog power generated from the amplified digital signal, whereby the quality of the signal can be improved. Such an arrangement can be achieved with a class D output stage, which means a circuit that comprises at least a class D amplifier to process the signal.
FIG. 1 shows a block diagram of a prior-art class D output stage. The block diagram shown comprises a sigma-delta modulator 101, a class D amplifier circuit 102 and an LC filter 103. In prior-art solutions the digital word at the input IN, which here refers to a set of parallel bits generated from an incoming digital serial or parallel word or pulse-coded message, is fed to the sigma-delta modulator 101 which converts the word fed into the input IN into a pulse train in which the pulse density corresponds to the value of said word fed to the input IN. This pulse train is fed to the class D amplifier circuit 102 which processes the pulse train so as to amplify it in a predetermined manner. For example, pulses indicating high bits may be amplified either to the maximum operating voltage of the amplifier or to ground potential, depending on what has been determined. Pulses indicating low bits are amplified in a corresponding predetermined manner. The amplified pulse train is fed in accordance with the prior art to a low-pass filter, in this case LC filter 103. A driver circuit 102 feeds the signal through the filter 103 direct to a load. By means of the filter 103 it is possible at least partly to remove from the pulse train switching-frequency interference and quantization noise generated in particular in the sigma-delta modulator 101.
The prior-art solutions described above have some disadvantages. First, prior-art solutions are highly susceptible to supply voltage fluctuations. As the supply voltage fluctuates the output audio signal becomes distorted. The distortion is typically of the third order and, additionally, the supply voltage fluctuation results in a noise component that increases the output impedance. Second, the prior-art solutions typically do not have error correction by means of which the disturbances caused by the power stage and external load could be mitigated and removed. If there is error correction, it is based on the use of analog signals. Furthermore, in prior-art solutions the linearity of the output signal of the power stage is not accurate enough for all applications. This is especially problematic in cases in which the supply voltage is unregulated so that it may fluctuate to a great extent.
An object of this invention is to reduce the above-described problems associated with the prior art and to provide an output stage by means of which the quality of an audio signal can be improved.
The objects of the invention are achieved by providing an audio output stage with feedback realized by digital signals, by means of which feedback the quality of a signal can be improved by digitally controlling the operation of a modulating circuit.
The method according to the invention for improving the quality of the output signal of an output stage, which comprises at least a modulator circuit, is characterized in that
a signal generated in the output stage, which signal is proportional to a previous digital input signal, is compared by means of feedback to the digital input signal of the output stage in order to generate a digital control signal, and
the operation of said modulator circuit is controlled by means of said digital control signal.
The output stage according to the invention for improving the quality of an output signal, which output stage comprises
a modulator circuit for modulating a digital input signal,
an amplifier circuit for amplifying the modulated signal,
a filter circuit for filtering the modulated and amplified signal,
is characterized in that the output stage further comprises a comparator circuit for comparing the digital input signal and a signal generated in the output stage, which signal is proportional to a previous digital input signal, and for generating a digital control signal for the modulator circuit.
The mobile station according to the invention, which mobile station comprises an output stage for processing a received audio signal, is characterized in that the output stage in the mobile station comprises
a first means to modulate a digital signal,
a second means to amplify a modulated signal,
a third means to filter a modulated and amplified signal,
a fourth means to generate a digital control signal by comparing the input signal to a signal generated in the output stage, which signal is proportional to a previous digital input signal, and by processing the signal which is the result of the comparison in such a manner that it is converted into a digital control signal suitable for the first means.
Advantageous embodiments of the invention are presented in the dependent claims.
In the arrangement according to the invention feedback is arranged at the output stage. By means of the feedback it is possible to improve the quality of the output signal of the output stage. The output stage advantageously comprises a modulator circuit, amplifier circuit, filter circuit and a comparator circuit. The input signal is modulated, after which the signal is amplified and filtered. Advantageously the output signal of the amplifier circuit is brought by the feedback to the comparator circuit where the signal is compared to the current input signal of the output stage. In accordance with a second arrangement the feedback is taken from the filter circuit output. On the basis of said comparison a difference signal is advantageously generated and then processed so as to provide a control signal suitable for the modulator circuit. The control signal is used to change the settings in the modulator circuit, whereby errors generated in the processing of the previous input signal are taken into account in the modulation of the input signal. Said feedback signal is made digital for the modulator circuit.